1. Field of the Invention
The present invention generally relates to the manufacture of holograms and, more specifically, to an improved method for monitoring the intensity during exposure of an exposure beam incident upon a hologram recording assembly.
2. Description of the Prior Art
In the production of hologram, such as those used for head-up displays (HUDs), helmet-mounted displays, and other pictorial-type holograms, such as those that might be used for automotive displays, it becomes particularly important to ensure that a wavefront with a specified intensity distribution be provided. Of course, as the need for higher quality holographic image increases, the need to monitor intensity increases. An ideal intensity monitoring system should be capable of operation prior to, during, and even after the exposure of the photosensitive film in the hologram recording assembly. Such a system enables monitoring during the most critical period--exposure--as well as during the times that the exposure can be adjusted. Further, an ideal system will not alter the exposure beam as a result of monitoring.
A prior monitoring system has been able to monitor the intensity of an exposure beam prior to the beam passing through a spatial filter in front of the photosensitive film. This method, however, accounts for total intensity but does not account for unevenness in the exposure beam, since the measured beam is unexpanded. Because there is considerable change in uniformity when the beam passes through the filter, the measurement has limited utility.
Another past method limits the monitoring to before and after exposure. Since the monitoring detectors are located on a light shutter, only when the shutter is closed will the detectors be in an operational position.
Yet another past method uses a beam splitter to reflect a portion of the exposure beam into detectors. This, however, suffers from the fact that the beam splitter cannot be formed with an optical shape to prevent distortion of the beam and, at the same time, reflect the beam to detector. To prevent distortion, the beam splitter would need outer surfaces with a point source at the center of curvature of the surfaces. That, however, would be impractical because, if an outer surface is reflective, the beam will reflect back on itself.
Another past system monitors the exposure beam in a single beam system after the exposure beam hits the photosensitive film. The drawback in this system is the need for two systems, since measurement is only being taken during the exposure itself and corrections must be made for absorption by the film.
In another system, a scanning system monitors the brightness of the hologram being created. The scanning system uses a light source of a wavelength not absorbed by the film and thus not reactive with the film. A chosen angle of incidence produces a diffracted beam that can be monitored. However, this is inadequate when both brightness of the beam creating the hologram and brightness of the hologram formed need to be monitored.
A need still exists in the art to provide an improved system for monitoring intensity during holographic exposure.